1. Field of the Invention
The present invention relates to an actuator, and more particularly to an improvement of an actuator having a cylindrical shape.
2. Related Background Art
A stepping motor having a reduced diameter about a rotation axis and an enhanced output level is disclosed in Japanese Patent Application Laid-open No. H09-331666. FIG. 1 shows an exploded perspective view of a stepping motor disclosed in Japanese Patent Application Laid-open No. H09-331666. FIG. 2 shows a sectional view of an assembled stepping motor taken along its axis.
FIGS. 1 and 2 show a rotor 101 having a cylindrical shape which is formed of permanent magnet (simply referred to as “magnet” in Japanese Patent Application Laid-open No. H09-331666) divided into four parts in a peripheral direction so as to alternately magnetize the parts into different polarities, a first coil 102 arranged adjacently to the rotor 101 in an axial direction, a second coil 103 similarly arranged adjacently to the rotor 101 in the axial direction, a first stator 104 that is formed of a soft magnetic material and excited by the first coil 102, and a second stator 105 that is formed of a soft magnetic material and excited by the second coil 103.
The first stator 104 includes first outside magnetic pole portions 104A and 104B that are opposed to an outer peripheral surface of the rotor 101 so as to have a clearance therebetween and first inside magnetic pole portions 104C and 104D that are opposed to an inner peripheral surface of the rotor 101 so as to have a clearance therebetween. The second stator 105 includes second outside magnetic pole portions 105A and 105B that are opposed to the outer peripheral surface of the rotor 101 so as to have a clearance therebetween and second inside magnetic pole portions 105C and 105D that are opposed to the inner peripheral surface of the rotor 101 so as to have a clearance therebetween.
The rotor 101 is fixed to an output shaft 106. The output shaft 106 is rotatably supported by a bearing portion 104E of the first stator 104 and a bearing portion 105E of the second stator 105. The first stator 104 and the second stator 105 are held by a coupling ring 107 formed of a non-magnetic material so as to have a predetermined clearance between the two stators.
With the above arrangement, a current flowing through the first coil 102 is reversed in direction to switch the polarities of the first outside magnetic pole portions 104A and 104B and the polarities of the first inside magnetic pole portions 104C and 104D, and a current flowing through the second coil 103 is similarly reversed in direction to switch the polarities of the second outside magnetic pole portions 105A and 105B and the polarities of the second inside magnetic pole portions 105C and 105D. This process is repeated to rotate the rotor 101.
In the stepping motor described above, magnetic flux lines generated by causing currents to flow through the coil pass from the outside magnetic pole portion to the inside magnetic pole portion opposed thereto or from the inside magnetic pole portion to the outside magnetic pole portion opposed thereto, thereby acting efficiently on each magnet composing the rotor arranged between the outside magnetic pole portion and the inside magnetic pole portion. Further, a distance between the outside magnetic pole portion and the inside magnetic pole portion that are opposed to each other can be set as being approximately as large as the thickness of the magnet having a cylindrical shape, thereby making it possible to reduce the resistance of a magnetic circuit formed by the outside magnetic pole portion and the inside magnetic pole portion. As the resistance of the magnetic circuit becomes smaller, more magnetic flux lines can be generated with a smaller amount of current, resulting in the enhanced output level.
Alternatively, Japanese Patent Application Laid-open No. H10-229670 discloses a structure of a motor obtained by further improving the above stepping motor. In the structure, an inside magnetic pole portion is formed into a cylindrical shape, an output shaft inserted into an inner portion of the inside magnetic pole portion is formed of a soft magnetic material and attached to a stator (composed of the inside magnetic pole portion and the outside magnetic pole portion), and a bearing that rotatably supports the output shaft is formed of a soft magnetic material. According to the proposed structure, the output shaft is also included in a magnetic circuit, enhancing the output of the motor. Adsorption between the stator and the output shaft, which may occur due to magnetism in the proposed structure, is prevented by forming the bearing of a, non-magnetic material.
However, the motor disclosed in Japanese Patent Application Laid-open No. H10-229670 has a problem in that magnetic flux lines generated by causing a current to flow through a first coil adversely affects a second coil, a second outside magnetic pole portion, and a second inside magnetic pole portion via the output shaft formed of a soft magnetic material, and magnetic flux lines generated by causing a current to flow through a second coil adversely affects a first coil, a first outside magnetic pole portion, and a first inside magnetic pole portion via the output shaft formed of a soft magnetic material, resulting in unstable rotation.
The motors disclosed in Japanese Patent Application Laid-open No. H09-331666 and Japanese Patent Application Laid-open No. H10-229670 each have a problem in that a predetermined clearance is necessary between the inner periphery of the magnet and the inside magnetic pole portion opposed thereto, and management of the clearance during manufacture causes an increase in manufacturing cost. There is another problem in that the stator needs to include the inside magnetic pole portion and the outside magnetic pole portion that are formed into the cylindrical shape, and it is difficult to integrally structure those portions in terms of manufacturing processes for parts. Further, there is still another problem in that in the case where those portions are separately manufactured and then integrally assembled, the number of necessary parts becomes large, causing another increase in manufacturing cost.
Further alternatively, a light quantity controller using a stepping motor that is driven on the same principle as the above-mentioned stepping motor is proposed in Japanese Patent Application Laid-open No. 2002-049076.
FIG. 3 shows an exploded perspective view of the actuator solely extracted from the light quantity controller disclosed in Japanese Patent Application Laid-open No. 2002-049076. FIG. 4 shows a sectional view in the case where the actuator of FIG. 3 is assumed to be cut along its axis.
FIGS. 3 and 4 show a magnet 201 having a cylindrical shape whose outer peripheral surface is divided into four parts in a peripheral direction and alternately magnetized into S poles and N poles and which can be rotated around the center of rotation, and a coil 202 having a cylindrical shape which is arranged in the axial direction of the magnet 201. The coil 202 excites a stator 203 that includes in its tip portion an outside magnetic pole portion 203a having a shape of a tooth and includes an inner cylinder 203b having a column-shaped portion in its tip. The outside magnetic pole portion 203a and the inner cylinder 203b are respectively opposed to the outer peripheral surface and the inner peripheral surface of the magnet 201. An auxiliary stator 204 is fixed to the inner cylinder 203b of the stator 203, and the auxiliary stator 204 and the inner cylinder 203b compose an inside magnetic pole portion. A base plate 205 includes a guide slot 205a engaged with a drive pin 201d that is provided to the magnet 201.
The portions described above compose the actuator of the light quantity controller.
The magnet 201 includes the drive pin 201d engaged with the guide slot 205a of the base plate 205, and shaft portions 201e and 201f such that the magnet 201 is rotatably supported by the base plate 205 and the stator 203. The magnet 201, the guide slot 205a, and the shaft portions 201e and 201f are unitarily molded. The outside magnetic pole portion 203a of the stator 203 is opposed to the outer peripheral surface of the magnet 201 so as to have a clearance therebetween. Similarly, the inside magnetic pole portion (composed of the inner cylinder 203b of the stator 203 and the auxiliary stator 204) is opposed to the inner peripheral surface of the magnet 201 so as to have a clearance therebetween.
With the actuator having the above-mentioned arrangement, currents flowing through the coil 202 is reversed in direction to switch the polarities of the outside magnetic pole portion 203a and the inside magnetic pole portion (the inner cylinder 203b and the auxiliary stator 204). This process is repeated to reciprocate the magnet 201 within a regulated range. Note that the rotational regulation for the reciprocating magnet 201 is performed by the guide slot 205a provided to the base plate 205 and the drive pin 201d engaged with the guide slot 205a. 
In the actuator described above, magnetic flux lines generated by causing a current to flow through the coil pass from the outside magnetic pole portion 203a to the inside magnetic pole portion opposed thereto or from the inside magnetic pole portion to the outside magnetic pole portion 203a opposed thereto, thereby acting efficiently on the magnet 201 placed between the outside magnetic pole portion 203a and the inside magnetic pole portion. Further, the distance between the outside magnetic pole portion 203a and the inside magnetic pole portion that are opposed to each other can be set as being as large as the thickness of the magnet 201 having a cylindrical shape with the clearance between the magnet 201 and the outside magnetic pole portion 203a and the clearance between the magnet 201 and the inside magnetic pole portion being added thereto, thereby making it possible to reduce the resistance of a magnetic circuit formed by the outside magnetic pole portion 203a and the inside magnetic pole portion. As the resistance of the magnetic circuit becomes smaller as described above, more magnetic flux lines can be generated with a smaller amount of current, resulting in the enhanced output level.
According to the above-mentioned actuator, as described above, the distance between the outside magnetic pole portion 203a and the inside magnetic pole portion that are opposed to each other can be set as being as large as the thickness of the magnet 201 having a cylindrical shape with the clearance between the magnet 201 and the outside magnetic pole portion 203a and the clearance between the magnet 201 and the inside magnetic pole portion being added thereto, thereby making it possible to reduce the resistance of the magnetic circuit.
However, in the above arrangement, the magnet 201 secures predetermined clearances with respect to the outside magnetic pole portion 203a and the inside magnetic pole portion, respectively. If one of the clearances is eliminated, the distance between the outside magnetic pole portion 203a and the inside magnetic pole portion becomes smaller by the width of the clearance, so that further reduction can be expected for the resistance of the magnetic circuit. Note that it can be taken into consideration to reduce the diameter of the actuator and to reduce the radial thickness of the magnet 201 having a cylindrical shape in order to reduce the distance between the outside magnetic pole portion 203a and the inside magnetic pole portion. However, such reduction causes a problem with the strength of the magnet 201, making it difficult to employ a structure merely with the magnet 201 having a smaller thickness. In addition, in the case where a predetermined clearance is necessary between the inner diameter of the magnet 201 and the inside magnetic pole portion opposed thereto as in Japanese Patent Application Laid-open No. 2002-049076, management of the clearance is also necessary during manufacture, causing an increase in manufacturing cost.